Accumulating evidence supports the concept that loss of growth control of cancer cells involves perturbation of signaling pathways that in the normal cell are controlled by growth regulatory factors. Platelet-derived growth factor (PDGF) is a connective tissue cell mitogen that has been implicated in tumorigenesis (Ostman and Heldin, Adv. Can. Res., 80:1–38, 2001, and references therein) as well as a principal player in the complex process of angiogenesis that is critical for tumor growth.
PDGF exerts its cellular effects through binding to its specific receptor, PDGF-R. PDGF-R is a transmembrane receptor tyrosine kinase (RTK). It consists of two isozymes α and β. Each of these receptors has an extracellular part featuring five immunoglobulin-like domains and an intracellular part with a tyrosine kinase domain. Both α and β-containing receptors have been associated with mitogenic activity, stimulating edge ruffling and loss of stress fibers. Only the β-containing receptors has been associated with chemotaxis and actin reorganization (Heldin, C -H, EMBO Journal 11:4251–4259,1992).
Binding of PDGF to PDGF-R results in dimerization of the two subunits of the PDGF-R receptors, thereby allowing autophosphorylation of each subunit on specific tyrosine residues in the cytoplasmic domain. The autophosphorylation leads to increased kinase activity and produces docking sites for a large number of signaling molecules with SH2 domains, such as Grb2/Sos1, PLC-gamma, GAP, PI-3 kinase and Src. (Heldin et al., Biochem. Biophys. Acta 1378: F79–113, 1998, and references therein). Upon binding to PDGF-R, these SH2 domain-containing signaling molecules initiate signal transduction pathways that are involved in different cellular responses such as cell proliferation, cell mobility, cell permeability or apoptosis.
PDGF has several important cellular effects in vivo. It regulates cell growth, differentiation, and migration during embryonal development, and plays a possible role in neuroprotection and regeneration. It also stimulates wound healing in adults. In addition, PDGF also has specialized functions in the vascular system as well as in the homeostasis of connective tissue (Ostman and Heldin, Adv. Can. Res., 80:1–38, 2001, and references therein).
Overactivity of PDGF has been implicated in the pathogenesis of a number of serious diseases, including cancers (glioma, lung, breast, colorectal, prostate, gastric and esophageal, leukemias and lymphomas), and other cell proliferative disorders, such as atherosclerosis, transplantation-induced vasculopathies, neointima formation, lung fibrosis, restenosis, pulmonary fibrosis, glomerulonephritis, glomerulosclerosis, congenital multicystic renal dysplasia, kidney fibrosis, and rheumatoid arthritis (Ostman A, Heldin C H., Adv. Cancer Res, 80:1–38,2001, and references therein).
A considerable body of direct and indirect experimental evidence showed that sustained tumor growth and metastasis are angiogenesis-dependent (see e.g., Hanahan, Science, 277:48–50, 1997). Angiogenesis is the development of new vasculature from preexisting blood vessels and/or circulating endothelial stem cells (see i.e., Springer et al., 1998). Angiogenesis plays a vital role in many physiological processes, such as embryogenesis, wound healing and menstruation. Angiogenesis also appears to be important in certain pathological events, such as solid tumor growth and metastasis, arthritis, psoriasis and diabetic retinopathy (Hanahan and Folkman, Cell, 86(3):353–364, 1996; ).
Anti-angiogenic therapy is currently being studied as a way to interfere with tumor growth. Indeed, numerous studies in animal models have demonstrated striking effects in tumor growth inhibition by targeting angiogenic growth factors such as vascular endothelial growth factor (VEGF), acidic and basic fibroblast growth factor (aFGF, bFGF) and PDGF. The receptors for VEGF and PDGF belong to one super family of receptor tyrosine kinases. Therefore, in addition to their role in treating other cell proliferative disorders, clinically useful PDGF-R tyrosine kinase inhibitors are useful for antiangiogenic therapy and to control tumor cell proliferation.
Small molecule inhibitors of the receptor tyrosine kinase constitute a novel class of drugs with large potential (Druker and Lydon, J. Clin. Invest., 105:3–7, 2000, and references therein). Since 1995, a number of small molecule inhibitors for PDGF receptor autophosphorylation have been characterized. Some examples are listed below.
JP 06087834 (Zimmermann) discloses N-phenyl-2-pyrimidine-amine derivatives which have tumor inhibitory activity and are useful for treating tumors in warm-blooded animals including human beings. Derivatives of this group of compounds, compound CGP53716 (Buchdunger et al., PNAS, 92:2558–2562, 1995) and compound STI-571 (Buchdunger et al., Cancer Res, 56:100–4, 1996), have been shown to inhibit PDGF-R autophosphorylation.
JP 11158149 (Kubo et al.) discloses quinoline derivatives for the treatment of diseases such as tumors and diabetic retinopathy. Derivatives of this group of compounds, compound Ki6783 (Yagi et al., Exp. Cell Res. 243:285–292, 1997) and compound Ki6896 (Yagi et al., Gen. Pharmacol. 31:765–773, 1998), have been shown to inhibit PDGF-R autophosphorylation.
U.S. Pat. No. 5,932,580 (Levitzki et al.) discloses PDGF receptor kinase inhibitory compounds of the quinoxaline family including Tyrphostin, ATP-competitive inhibitors of the receptor kinase.
U.S. Pat. No. 5,409,930 (Spada, et al.) discloses bis mono- and/or bicyclic aryl and/or heteroaryl compounds exhibiting protein tyrosine kinase inhibition activity. Compound RPR101511A, a derivative of this group of compound, has been shown to inhibit PDGF-R autophosphorylation (Bilder et al., Circulation. 99(25):3292–9. 1999).
U.S. Pat. No. 5,563,173 (Yatsu, et al.) discloses a method of inhibiting the proliferation of smooth muscle cells by sodium butyrate, which inhibits PDGF-R kinase activity.
U.S. Pat. No. 5,476,851 (Myers, et al.) discloses Pyrazolo[3,4-g]quinoxaline compounds, as PDGF receptor protein tyrosine kinase inhibitors.
Compound SU-6668, an ATP competitive inhibitor, has been shown to inhibit PDGF-R autophosphorylation (Laird, et al., Cancer Res. 60:4152–4160, 2000].
WO01/79198 (Reich et al.) discloses amino-pyrazole compounds of the following formula that modulate and/or inhibit the activity of protein kinases.

WO0212242 (Fancelli et al.) discloses bicyclo-pyrazole compounds that are useful for treating diseases linked to disregulated protein kinases.
Up to now, STI-571 (GLEEVEC) is the only compound to reach market with significant PDGFR activity, although it is not a selective antagonist of this enzyme. Therefore, PDGF-R remains an extremely attractive target for the design of potent and selective small molecule inhibitors that will represent an important new class of therapeutic agents for the treatment of tumors and other cell proliferative disorders.
References to a number of substituted tricyclic pyrazole derivatives include those disclosing use as: inhibitors of tyrosine kinase activity (WO 99/17769, WO 99/17770); cyclin dependent kinases inhibitors (WO 99/54308); selective estrogen receptor modulators (WO 00/07996); analgesics (U.S. Pat. No. 4,420,476); prophylaxis and therapy of diseases caused by rhinoviruses (U.S. Pat. No. 4,220,776; U.S. Pat. No. 4,140,785); analgesics/anti-inflammatory activity (U.S. Pat. No. 3,928,378; Schenone, Silvia et al. Farmaco (2000), 55(5), 383–388); cyan couplers for photographic dye (EP 0620489, JP 8022109); quinolines and naphthyridines as drugs (JP 6092963); and immunomodulators (JP 6100561); and hypoglycemic agents (Reddy, R. Raja et al., Indian Journal of Heterocyclic Chemistry (1998), 7(3), 189–192).